Sky & Telescope - April 2021 - 53

HERODOT US A ND A RISTA RCHUS: N ASA / GSFC / A RIZON A STATE
UNIV ERSIT Y / CH A RLES WOOD (2); JA NSEN: N ASA / GSFC /
A RIZON A STATE UNIV ERSIT Y

too could have been ﬁlled with about
2 km thickness of lava. Only the top
1.2 km of Aristarchus's rim would be
visible, and its 300-meter-high central
peak would be deeply buried beneath a
smooth lava surface. This is what happened at Plato and with other skatingrink craters - they were inundated
by so much rising magma that their
original ﬂoors, lower walls, and central
peaks were submerged.
Not all craters are ﬁlled so deeply
with lava. Hercules, at the east end
of Mare Frigoris, has a smooth ﬂoor
with one signiﬁcant disruption - the
14-km-wide crater Hercules G. At 68

km wide and 3.5 km deep, Hercules
has a shallow lava pool that didn't even
cover its small peaks rising 300-400
meters above its existing ﬂoor. Because
of its great depth, and peaklets, I
wouldn't call Hercules a skating rink.
Similarly, smooth-ﬂoored Neper (130
km) wouldn't qualify. It's 3.9 km deep
and has a central peak that rises 2.2
km above the lava-covered ﬂoor. You'll
have to take your skates elsewhere.
Two of the more remarkable lunar
skating rinks are Crüger and Billy.
These 46-km-wide craters formed
with depths of roughly 3 km but now
contain thick lava - 2.3 km in Crüger

Jansen

S Unlike the other skating-rink craters, Jansen
appears to be a completely volcanic formation
and not the result of an impact.

and 1.9 km in Billy - burying their
peaks and most of their walls. Billy is
at the southernmost range of Oceanus Procellarum, but Crüger seems
to be far from a source of mare lava.
Yet looks can be deceiving. Gravity
data derived from NASA's Clementine
mission revealed an otherwise totally
undetectable impact basin as large as
the Grimaldi Basin immediately to the
north. Crüger and other nearby small
patches of mare lava lie inside this
Crüger-Sirsalis Basin.
All the skating-rink craters discussed
so far are volcanically modiﬁed impact
craters. However, Jansen (24 km) is a
skating rink that may be a completely
volcanic structure. It resides on a
slightly elevated ridge that's about 800
meters above other nearby lava plains to
the north. The ridge is cut by a sinuous rille and peppered with rimless
pits - all of which are volcanic features.
Jansen, like Crüger, is on the edge of a
positive gravity anomaly, hinting that a
small basin may be buried nearby.
If there's one feature we can consider
a " mega-skating rink, " it's Mare Crisium. It's 400 km wide and lies 4 to 5.5
km lower than the Crisium Basin rim.
The mare has no central mountains -
just a few small craters and low ridges
to slow your skating.

S Top: Craters Herodotus (left) and Aristarchus (right) are nearly the same size, though Aristarchus
formed long after the final mare eruptions. Above: Had Aristarchus formed 3 billion years earlier, it
likely would appear much as it does in this simulation.

After 20 years, Contributing Editor
CHUCK WOOD continues to piece together fascinating lunar stories to share
with S&T readers.
sk yandtelescope.org * APRIL 2 021

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Sky & Telescope - April 2021

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